The field of this invention is blood mass transfer devices, particularly oxygenators, wherein some desirable constituent (e.g., oxygen) is transferred into the blood and/or some undesirable constituent (e.g., carbon dioxide) is transferred out of the blood. Three basic types of oxygenators have developed over time: film oxygenators (e.g., U.S. Pat. No. 3,070,092); bubble oxygenators (e.g., U.S. Pat. Nos. 3,915,650 and 4,428,934); and membrane oxygenators (e.g., U.S. Pat. No. 4,698,207).
Film oxygenators are characterized by exposing a continuous thin film of blood to an oxygen atmosphere. The surface upon which the blood is filmed must be chemically inert and not damage the blood. Additionally, the surface must sustain a very thin film in order to maximize the diffusion of oxygen into the blood. In bubble oxygenators, oxygen is introduced into the blood as bubbles which oxygenate the blood and drive off carbon dioxide. In these oxygenators, the bubbling or foaming mixture must be passed through a "defoamed" to eliminate gas bubbles from the oxygenated blood before it is returned to the patient. In a typical membrane oxygenator, blood is carried in or around hollow membrane fibers. Oxygen passes through the membrane from an oxygen-rich gas stream to the bloodstream, and carbon dioxide passes through the membrane from the blood to the gas stream. The number and size of the hollow membrane fibers are selected to transfer sufficient oxygen to satisfy the metabolic requirements of the patient. Before the blood is returned to the patient from the membrane oxygenator, it is usually passed through a filter to remove any particulate emboli or gas bubbles. The filter is usually in the arterial line outside of the oxygenator itself.
Various types and configurations of foam have been used for specific purposes in bubble and film oxygenators. Blood oxygenators which use foam material to "defoam" the blood-oxygen mixture, i.e., remove bubbles from the blood, are well known as illustrated by the blood oxygenator in U.S. Pat. No. 4,158,693 to Reed et al. Foam material is also used in the Reed et al. bubble oxygenator to provide an enlarged surface area for oxygen-blood contact, and to disperse the blood so it will rise uniformly through the oxygenating chamber. The film oxygenator in U.S. Pat. No. 3,070,092 to Wild et al. uses a porous sponge material as the surface on which the blood is filmed. None of these types of oxygenators contemplates using a foam material as both the blood pathway and the membrane across which oxygenation occurs.
Certain parameters must be considered when designing an oxygenator, whether of the film, bubble, or membrane type. Parameters which must be considered include the overall size and geometry of the oxygenator, blood volume that can be oxygenated, damage to the blood, the rate of gas exchange, and the volume of blood physically held by the oxygenator (known as "priming volume").
The physical size of an oxygenator is determined in large part by the effective exchange surface area, that is, the exchange surface area the blood is exposed to for oxygenation. The total volume of blood that can be oxygenated must be sufficient to satisfy the metabolic requirements of a patient. As discussed in U.S. Pat. No. 4,698,207 to Bringham et al., this can require using 41,000 to 71,000 hollow fibers in a hollow fiber membrane oxygenator. In order to minimize the size of a blood oxygenator, a large exchange surface area must be contained in a small volume. As a result, the exchange surface area may have to assume intricate geometries which is made difficult by the structures of conventional membrane oxygenators. Intricate geometries are also difficult to achieve with conventional film and bubble oxygenators, as illustrated by the grid of plates in the film oxygenator in U.S. Pat. No; 3,070,092 to Wild et al. and the aluminum oxygenator tubes in U.S. Pat. No. 4,280,981 to Harnsberger.
Blood is a very delicate body tissue and is damaged when handled and exposed to foreign surfaces and gas atmospheres. Requiring the blood to flow through or around fibers or through tubes composed of substances such as aluminum or styrenes physically damage the blood by denaturation of proteins and mechanical damage to cells and formed elements.
In film and bubble oxygenators, the oxygen diffuses directly into the blood from the oxygen-rich atmosphere; carbon dioxide diffuses out of the blood to that atmosphere. In the membrane oxygenator, the oxygen and carbon dioxide diffusion take place across a permeable membrane. The design of the oxygenator, e.g., choice of membrane material, should maximize the rate of gas exchange, that is, the rate of absorption of the oxygen by the blood without exposing the blood directly to a gas atmosphere.
It is apparent that a blood oxygenator which maximizes the rate of gas exchange may require a large exchange surface area and oxygenator volume, and may also damage the blood. The design parameters conflict such that optimizing one parameter may degrade another. Therefore, the problem remains to optimize all the parameters to design a blood oxygenator that has a large exchange surface area per unit volume, can take on different geometries, minimizes damage to the blood, and maximizes the rate of gas exchange.
The same conflicting parameters exist for other mass transfer devices, such as dialyzers. Dialyzers perform the function of removing metabolic waste products without removal of essential constituents such as proteins. The problem to be solved here, analogous to that of blood oxygenators, is to design a dialyzer which has a large exchange surface area per unit volume, can take on different geometries, minimizes damage to the blood, and maximizes the rate of removal of the waste products from the blood.
Accordingly, prior to the development of the present invention, no single blood mass transfer device provided a large exchange surface area in a small volume capable of different geometries, and which minimized damage to the blood while providing a high rate of mass transfer. It is therefore an object of the present invention to provide a mass transfer device which has a large exchange surface area in a small volume, and which minimizes damage to the blood while achieving a high rate of mass transfer. It is a further object of this invention to provide a blood oxygenator which has a large exchange surface area in a small volume, and which minimizes damage to the blood while achieving a high rate of gas exchange. It is a further object of this invention to provide a blood dialyzer which has a large exchange surface area in a small volume, and which minimizes damage to the blood while achieving a high rate of molecular transport. It is a feature of this invention to use a pliable foam material in the mass transfer device as both the blood pathway and the membrane across which the transfer occurs. It is an additional feature of this invention that the blood mass transfer devices can take on varied and intricate geometries to satisfy the requirements of the particular application.